1. Field of the Invention
The present invention relates to a numerical controller for controlling feed movements of a wheel head with a grinding wheel and a work table of an angular slide grinding machine in accordance with numerical control data so that the grinding wheel can be dressed with a dressing tool fixedly provided on the work table.
2. Description of the Prior Art
Generally, conventional angular slide grinding machines have a machine origination position, which is defined by an end surface of a grinding wheel when a cylindrical surface of the grinding wheel perpendicular to the end surface reaches the axis of a workpiece rotatably carried on a work table by means of a pair of centers. The axial position of the workpiece relative to the grinding wheel is controlled by moving the work table in a machine coordinate system that originates from the original machine position or a table origination position spaced a predetermined distance therefrom in the moving direction of the work table. Center holes of workpieces of each lot have an inequality in their depths relative to reference surfaces of the workpieces. The inequality in the center hole depths causes precise positioning of each workpiece relative to the grinding wheel to be difficult, thereby degrading finish accuracy of the workpieces. Accordingly, feed movement of each workpiece relative to the grinding wheel must be compensated in order to provide for the difference between an actual size and a designed size of the center hole depth of the workpiece.
The prior art uses two methods of providing for such compensation. In a first method, an end surface sizing device is mounted on the work table, and a difference between an actual position and a designed position of the reference surface of a workpiece supported on the work table, that is a difference between an actual and designed depths of the center holes of the workpiece, is detected by the sizing device so as to compensate commanded data for the detected difference. This method requires adjustment of the mounting position of the sizing device each time the type of workpiece being machined is changed, which results in imposing a limitation upon the flexibility of the grinding machine because the adjustment of the mounting position relies on human power. The compensation of the commanded data for the detected difference requires the use of an analogue-to-digital converter. Further, there exists a prior art headstock and a tailstock which are capable of axially moving their centers to bring the reference surface of a workpiece carried thereby into a predetermined positional relation with a worktable. However, such a headstock and such a tailstock have complicated constructions and thus raise manufacturing costs.
The second prior art method employs an end surface sizing device fixedly mounted on a bed of the grinding machine. The work table is moved to bring the reference end surface of the workpiece into engagement with a probe of the sizing device and is stopped when the reference end surface reaches a sizing point. This method is used in various types of angular slide grinding machines presently in operation because it has the advantage that a single sizing device can control relative position between the grinding wheel and each of workpieces which have their reference surfaces at different axial positions and because the mounting position of the sizing device need not be adjusted once it is mounted on the bed.
In the second method, a position of the work table where the reference surface of a workpiece reaches the sizing point of the sizing device is chosen as a positioning reference for feed movements of the work table in a subsequent grinding operation of the workpiece. A coordinate value of the workpiece reference surface relative to the machine original position is preset in a table present position counter in advance of the grinding operation, and thus, a workpiece coordinate system is established for feed control of the work table in the grinding operation. The workpiece coordinate system is different in position from the machine coordinate system that originates from the machine origination position or the table origination position spaced a predetermined distance therefrom, and the positional difference therebetween corresponds to the difference between the actual and designed center hole depths of each workpiece.
In dressing the end surface and the cylindrical surface of the grinding wheel with a dressing tool fixedly provided on the work table, the positional relation between the grinding wheel and the dressing tool is determined by the position of the work table relative to the grinding wheel. In order to realize precise grinding wheel dressing, therefore, the relative movement between the grinding wheel and the dressing tool (i.e., the work table) must be controlled in the machine coordinate system which defines the position of the work table relative to the machine original position. After the completion of a machining operation, however, the workpiece coordinate system is established and maintained, and the grinding wheel dressing operation is, as a matter of fact, impossible to perform with the workpiece coordinate system being held constant.
For the reason mentioned above, when the dressing of the grinding wheel is needed between two successive grinding operations on the same workpiece in prior angular slide grinding machines, the work table must be returned to the table origination position to thereby establish the machine coordinate system again in advance of a subsequent one of the two successive grinding operations. In addition, the workpiece coordinate system must be re-established after the dressing operation by moving the work table to such a position as to set the reference surface of the workpiece to the sizing point of the sizing device.